Adaptive time slot allocation to reduce latency and power consumption in a time slotted channel hopping wireless communication network

ABSTRACT

Excessive latencies and power consumption are avoided when a large number of leaf nodes (LNs) contend simultaneously to join a time slotted channel hopping wireless communication network having a root node (RN) interfaced to LNs by one or more intermediate nodes (INs). A first plurality of shared transmit/receive slots (STRSs) is allocated for at least one IN, and a second plurality of STRSs is advertised for use by contending LNs, where the first plurality is larger than the second plurality. When a LN joins, its STRSs are re-defined such that most become shared transmit-only slots (STOSs) and no STRSs remain. The numbers of STRSs allocated to INs may vary inversely with their hop counts from the RN. One or more STOSs may be added for each of one or more INs in response to a predetermined network condition.

This application claims 35 USC 119 priority to copending US Provisional Application No. 62/169,876, filed on Jun. 2, 2015 and incorporated herein by reference.

FIELD

The present work relates generally to wireless communication networks that employ time slotted channel hopping and, more particularly, to techniques for accommodating increases in the number of nodes joining the network with reduced latency and power consumption.

BACKGROUND

The following documents are incorporated herein by reference:

-   -   IEEE STD 802.15.4; and     -   IEEE STD 802.15.4e.

FIG. 1 diagrammatically illustrates a simplified topology of a typical conventional wireless communication network operating in compliance with IEEE 802.15.4e, which is an amendment to the IEEE STD 802.15.4 radio communication protocol specification. This amendment specifically targets MAC protocol level modifications to enhance the performance of devices using IEEE 802.15.4 radios. A specific IEEE 802.15.4e MAC operation is referred to as Time Slotted Channel Hopping (TSCH), which enables robust as well as low-power communication. Channel hopping provides robustness against interference. The time slotted and time-synchronized nature of the protocol allows for time-scheduled communication. The scheduled communication means that the devices need to be active only when required and in “sleep” mode otherwise. This provides low-power operation.

As shown, the topology of FIG. 1 has a hierarchical structure with a plurality of hierarchical levels. The network of FIG. 1 includes at 11 a central root node (RN) that controls the network and serves as a gateway to higher bandwidth networks. A Level 1 intermediate node (IN) at 12 interfaces between the RN and a plurality of Level 2 INs at 13, which each in turn interfaces between the Level 1 IN 12 and one or more leaf nodes (LNs) at 14. The wireless communication links illustrated between the various nodes of FIG. 1 are also referred to herein as communication hops (or simply hops), such that, for example, RN 11 is three hops away from each LN at 14.

TSCH uses a sequence of time frames that are each divided into a plurality of slots of time (time slots). In the IEEE 892.15.4e scheduling scheme, each node is allocated a shared slot and at least one beacon slot in each time frame. A beacon slot is used by the RN 11 to transmit a beacon packet that contains the transmit/receive schedules of nodes in the network. Beacon slots are also used for time synchronization. The shared slot is a shared transit/receive slot (STRS) in which a node may either transmit or receive. New nodes attempting to associate with (join) the network contend for access to the STRS, and a successfully contending node transmits a network association request in the STRS. The STRS is the same interval of absolute time for each node of FIG. 1. The STRS is also used to convey network maintenance information and traffic routing information.

The RN 11 uses the STRS to transmit an association response in reply to an association request received from a successfully contending node. As new nodes join the network, they receive respectively corresponding association responses that allocate to them, in sequential fashion, respective pairs of dedicated slots. Each pair of dedicated slots consists of a transmit slot and a receive slot, both reserved for use only by the node to which they are allocated. If the joining node is an IN, the association response also allocates an additional beacon slot to the IN, to support the relay of beacon packets across the hops from the RN to the LNs.

A node operating in the network wakes up from sleep mode at every time slot and checks what function is to be performed. If none, it returns to the sleep state. The node listens in an active receive state during a beacon slot, an STRS and a dedicated receive slot, in order to ensure that no communication is missed. In a dedicated transmit slot, the node transmits as needed, or returns to sleep if it has nothing to transmit.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings referenced herein:

FIG. 1 diagrammatically illustrates a simplified topology of a conventional wireless communication network;

FIGS. 2A and 2B illustrate operations that can be performed in a network such as shown in FIG. 1 according to example embodiments of the present work;

FIGS. 3 and 4 diagrammatically illustrate allocations of shared slots according to example embodiments of the present work; and

FIG. 5 diagrammatically illustrates an apparatus for use as a root node in a network such as shown in FIG. 1 according to example embodiments of the present work.

DETAILED DESCRIPTION

When many LNs attempt to join the network of FIG. 1 simultaneously, they will all contend during the single STRS, and the INs use the STRS to relay the association requests and responses. The present work has recognized that these factors can disadvantageously result in high latencies and high power consumption during association due to the contention for the STRS, and because all nodes consume power during the STRS. Every node awakes for the STRS, actively contends for access to the STRS if transmission is desired, and then transmits if contention is successful. Also, if transmission is not desired, or if contention is not successful, the node still listens in the active receive state. The latency and power consumption problems will be further exacerbated by the aforementioned use of the STRS for communication of network maintenance information and traffic routing information.

The present work has also recognized that the aforementioned latency and power consumption problems might be addressed by providing additional STRSs. However, simply increasing the number of STRSs would significantly increase power consumption because, as indicated above, all nodes would need to be awake in each STRS.

Example embodiments of the present work apply strategic solutions to the latency and power consumption problems encountered as TSCH networks experience increased demand of nodes attempting to join the network. These solutions exploit factors such as: INs (which have relatively large batteries or power-harvesting capabilities) are typically less power-constrained than LNs (which have relatively small batteries); INs at higher levels of the hierarchy experience more traffic than INs at lower levels; the operating profile of an LN changes after it becomes associated with the network; and the post-association operating profile of the LN typically requires more uplink communication (to the RN) than downlink communication (from the RN). As described in detail below, the RN uses messages such as beacon packets and association responses to implement solutions according to example embodiments of the present work.

To alleviate the latency and power consumption problems, an RN according to example embodiments of the present work allocates for LN association contention a suitably-sized plurality of STRSs to help accommodate a large LN association contention load. The RN transmits a beacon message (beacon packet) that advertises the number of STRSs allocated for LNs to use in contending for association. All nodes desiring network association may then contend for association in each of the advertised STRSs.

In some embodiments, when the RN responds to a successful association request, its association response (transmitted across network hops in STRSs) re-defines the plurality of shared slots available to the newly associated LN, such that the shared slots comport with the post-assocation operating profile of the LN and provide for reduced LN power consumption. For example, LNs in sensor applications typically use uplink extensively for data transfer. Accordingly, some embodiments re-define the plurality of STRSs such that: at least one STRS is changed to a shared receive-only slot (SROS) in which the LN may only receive communications; and the rest (a majority) of the STRSs are changed to shared transmit-only slots (STOSs) in which the LN may only transmit communications. A SROS always requires power consumption for listening in the active receive state, whereas an STOS requires power consumption only when the node has information to transmit, in which case the node contends for the STOS and, if the contention is successful, transmits the information. Because the re-definition of the STRSs results in allocation of mostly STOSs to the LN, it comports with the LN's post-association operating profile (mostly uplink transmissions). Because the re-definition is complete, i.e., it retains none of the STRSs, it avoids unnecessary LN power consumption. In some embodiments, the re-definition changes only one STRS to an SROS, and changes the rest of the STRSs to STOSs. In some embodiments, the re-definition adds one or more further STOSs such that the LN is actually allocated a total number of SROS(s) and STOSs that is larger than the total number of STRSs that were initially available to the LN for contending to join the network.

Because INs also experience increased traffic as large numbers of LNs attempt to join the network, in some embodiments, the RN allocates a relatively large number of STRSs to an IN. The RN's association response to a joining IN is used in some embodiments to communicate this allocation of STRSs. In some embodiments, the number of STRSs allocated to an IN exceeds the number of STRSs advertised for use by LNs contending for network association. Because a IN is typically less power-constrained than a LN, the added power-consumption required by the relatively large number of STRSs allocated to the INs will generally be acceptable. Furthermore, because a higher level IN (e.g., Level 1 IN of FIG. 1) handles more traffic than an associated lower level IN (e.g., Level 2 IN of FIG. 1), more STRSs may be allocated for a higher level IN than for an associated lower level IN. Note that the RN knows the hierarchical level of a joining IN by inspecting the hop count conventionally provided in the association request received from the IN. The hop count is incremented at each hop traversed by the association request.

As network conditions change, it may be helpful to provide one or more INs with more network communication capacity. For example, as the number of LNs in the network increases, one or more INs may require more uplink capacity to accommodate increased uplink traffic from the LNs. In some embodiments, when the RN detects that the number of LNs in the network exceeds a predetermined threshold, it transmits a beacon message to add one or more STOSs (and/or one or more SROSs) to the shared slot allocation of each of one or more INs. Some embodiments limit the number of STOSs/SROSs added for an IN such that its resulting total allocation of STOSs and SROSs and STRSs does not exceed the number of STRSs allocated to any associated IN at the next higher level of the hierarchical topology.

In some embodiments, the aforementioned messages (i.e., beacon messages and association responses) used to implement solutions according to the present work employ payload information elements (IEs) that are conventionally available in those messages. The payload IEs are suitably formatted to indicate the location of the shared slots, how many shared slots are STOS, how many shared slots are SROS, and how many shared slots are STRS. In the instances where a beacon message advertises STRSs for association contention, the payload IE contains a predetermined broadcast identifier such that all potentially joining nodes are informed. In the other above-described instances of shared slot re-definition and shared slot allocation, the payload IE (whether in a beacon message or an association request) contains information that identifies the node to which the payload IE is directed.

FIGS. 2A and 2B illustrate operations described above that can be performed in a network such as shown in FIG. 1 according to example embodiments of the present work. The illustrated operations are performed by the RN. When an association request is received from an IN at 21 in FIG. 2A, the hop count in the request is inspected at 22. If the hop count=1, then the IN is joining at Level 1 (see also FIG. 1), so a total of A STRSs are allocated to the IN at 23. If the hop count=2, then the IN is joining at Level 2, so a total of B (B<A) STRSs are allocated at 24.

At 25 and 26 in FIG. 2B, there is illustrated the allocation of one or more STOSs (and/or one or more SROSs) to each of one or more INs, in response to detection of a predetermined condition in the network. In the example of FIG. 2B, the predetermined condition is the number of LNs in the network exceeding a threshold TH, as shown at 25. If the condition at 25 is satisfied, the aforementioned allocation of one or more STOSs (and/or one or more SROSs) to each of one or more INs occurs as shown at 26.

At 27 in FIG. 2B, there is illustrated advertisement of C (C<B) STRSs for LNs to use in contending for network association. If a LN association request is received at 28, the C STRSs advertised at 27 are changed for the new LN at 29, to become D SROS(s) and E STOSs, where D+E=C and D<E. As noted above, the re-definition at 29 may also include addition of one or more further STOS s (not explicitly shown in FIG. 2B). After the re-definition at 29, operations may proceed to check for the network condition at 25, unless that condition was already detected previously, in which case operations proceed to 27 as shown by broken line in FIG. 2B.

In some embodiments, the value of threshold TH is updated (increased) in response to a “yes” decision at 25 in FIG. 2B, thus providing for the possible allocation of further STOSs/SROSs (at 26) as the network grows. In such case, the broken line path from 29 to 27 in FIG. 2B would be omitted.

FIGS. 3 and 4 show bar graphs that illustrate example allocations of shared slots to a Level I IN, a Level 2 IN and a LN according to example embodiments of the present work. FIG. 3 shows allocations existing before the LN has joined the network, and FIG. 4 shows allocations after the LN joins. FIG. 3 shows A, B and C STRSs allocated for use by the Level I IN, the Level 2 IN and the LN, respectively, with A>B>C. FIG. 4 shows post-association shared slot re-definition for the LN such that the STRSs previously allocated to the LN (see FIG. 3) for association contention are changed to D SROS(s) and E STOSs, with D<E and D+E=C. FIG. 4 also shows, in broken line, F STOSs added to the shared slot allocation of an IN (Level 2 IN in this example) in response to detection of a predetermined condition in the network. As one particular illustrative example, some embodiments have A=10, B=6, C=4, D=1, E=3 and F=4. As another particular example, the LN's shared slot re-definition in some embodiments (not shown in FIG. 4) adds two further STOSs for the LN, resulting in a total of E+2 (e.g., 3+2=5) STOSs and C+2 total shared slots for the LN, with C+2<B<A.

FIG. 5 diagrammatically illustrates an apparatus for use as a RN in a TSCH wireless communication network such as shown in FIG. 1 according to example embodiments of the present work. Various conventional structures and functions not necessary for understanding the present work may be omitted. The apparatus of FIG. 5 is capable of performing operations described above and shown in FIGS. 2-4. A slot allocator 51 is coupled for communication with a node association interface 53 and a beacon generator 55. In some embodiments, the slot allocator 51, the node association interface 53 and the beacon generator 55 are collectively implemented by a suitably programmed data processor. As indicated diagrammatically by the broken line at 57, the node association interface 53 receives incoming association requests from nodes that are associating with the network, and outputs corresponding association responses for transmission through the network to the associating nodes. In some embodiments, the node association interface 53 uses conventional techniques to analyze each incoming association request, and then forwards to the slot allocator 51 pertinent information from the association request. For example, in some embodiments, the node association interface 53 forwards to the slot allocator 51 information such as the type of node (LN or IN) that sent the incoming association request, and the hop count contained in the association request. The slot allocator 51 determines the shared slot allocation for the associating node based on the association request information received from the node association interface 53, and forwards the determined shared slot allocation to the node association interface 53. The node association interface 53 prepares for the associating node an association response that contains the determined shared slot allocation, and then outputs the association response at 57 for transmission through the network to the associating node.

The beacon generator 55 receives shared slot allocation information from the slot allocator 51, prepares a beacon message that contains the received shared slot allocation information, and outputs the beacon message at 59 for transmission through the network. As described above, beacon messages are used to advertise STRSs allocated by the slot allocator 51 for use by nodes contending to associate with the network. As also described above, beacon messages are used to inform INs that STOSs and/or SROSs (allocated by slot allocator 51) are added to their shared slot allocations. In some embodiments, the slot allocator 51 maintains a count of the number of LNs in the network, and compares this count with a threshold to determine when to add STOSs/SROSs to the shared slot allocations of INs (see also 25 and 26 in FIG. 2).

Although example embodiments of the present work are described above in detail, this does not limit the scope of the present work, which may be practiced in a variety of embodiments. 

What is claimed is:
 1. A method of allocating slots of time in a time slotted channel hopping (TSCH) wireless communication network that includes at least one intermediate node (IN) that interfaces between a root node (RN) and a plurality of leaf nodes (LNs), comprising: allocating a first plurality of shared transmit/receive slots (STRSs) to an IN when the IN associates with the network; and advertising a second plurality of STRSs allocated as contention slots in which LNs may contend for association with the network; wherein the first plurality of STRSs is larger than the second plurality of STRSs.
 2. The method of claim 1, including, when a LN associates with the network, re-defining the second plurality of STRSs for the LN such that at least one of the STRSs is changed to a shared receive-only slot (SROS) and a remainder of the STRSs are changed to shared transmit-only slots (STOSs), wherein said remainder is a majority of the STRSs.
 3. The method of claim 2, including allocating a third plurality of STRSs to a further IN when the further IN associates with the network, wherein the third plurality of STRSs is larger than the second plurality of STRSs and smaller than the first plurality of STRSs, and wherein the IN interfaces between the further IN and the RN.
 4. The method of claim 3, including, after said first and third pluralities of STRSs are allocated, further allocating one of at least one STOS and at least one SROS to each of at least one of the IN and the further IN in response to detection of a predetermined condition in the network.
 5. The method of claim 2, including, after said allocating, further allocating one of at least one STOS and at least one SROS to the IN in response to detection of a predetermined condition in the network.
 6. The method of claim 1, including allocating a third plurality of STRSs to a further IN when the further IN associates with the network, wherein the third plurality of STRSs is larger than the second plurality of STRSs and smaller than the first plurality of STRSs, and wherein the IN interfaces between the further IN and the RN.
 7. The method of claim 6, including, after said first and third pluralities of STRSs are allocated, further allocating one of at least one STOS and at least one SROS to each of at least one of the IN and the further IN in response to detection of a predetermined condition in the network.
 8. The method of claim 1, including, after said allocating, further allocating one of at least one STOS and at least one SROS to the IN in response to detection of a predetermined condition in the network.
 9. A method of allocating slots of time in a TSCH wireless communication network that includes at least one IN that interfaces between a RN and a plurality of LNs, comprising: advertising a first plurality of STRSs allocated as contention slots in which LNs may contend for association with the network; and when a LN associates with the network, re-defining the plurality of STRSs for the LN such that at least one of the STRSs is changed to a SROS and a remainder of the STRSs are changed to STOSs, wherein said remainder is a majority of the STRSs.
 10. The method of claim 9, wherein only one STRS is changed to a SROS.
 11. The method of claim 9, including allocating a second plurality of STRSs to an IN when the IN associates with the network and, thereafter, allocating one of at least one STOS and at least one SROS to the IN in response to detection of a predetermined condition in the network.
 12. The method of claim 9, including allocating a second plurality of STRSs to an IN when the IN associates with the network, and allocating a third plurality of STRSs to a further IN when the further IN associates with the network, wherein the third plurality of STRSs is larger than the first plurality of STRSs and smaller than the second plurality of STRSs, and wherein the IN interfaces between the further IN and the RN.
 13. The method of claim 12, including, after the second and third pluralities of STRSs are allocated, further allocating one of at least one STOS and at least one SROS to each of at least one of the IN and the further IN in response to detection of a predetermined condition in the network.
 14. A method of allocating slots of time in a TSCH wireless communication network that includes at least one IN that interfaces between a RN and a plurality of LNs, comprising: allocating a plurality of STRSs to an IN when the IN associates with the network; and thereafter, allocating one of at least one STOS and at least one SROS to the IN in response to detection of a predetermined condition in the network.
 15. The method of claim 14, wherein the predetermined condition is more LNs than a predetermined threshold.
 16. A method of allocating slots of time in a TSCH wireless communication network that has a hierarchical topology with a plurality of hierarchical levels and a RN interfaced to a plurality of LNs by a first IN at a relatively higher hierarchical level and a second IN at a relatively lower hierarchical level, comprising: allocating a first plurality of STRSs to the first IN when the first IN associates with the network; and allocating a second plurality of STRSs to the second IN when the second IN associates with the network; wherein the first plurality of STRSs is larger than the second plurality of STRSs.
 17. The method of claim 16, including, after the first and second pluralities of STRSs are allocated, further allocating one of at least one STOS and at least one SROS to each of at least one of the first and second INs in response to detection of a predetermined condition in the network.
 18. An apparatus for use as a RN in a TSCH wireless communication network that includes at least one IN that interfaces between the RN and a plurality of LNs, comprising: a slot allocator configured to allocate slots of time; a node association interface coupled to said slot allocator and configured to produce, when an IN associates with the network, a message for transmission to the IN that indicates allocation of a first plurality of STRSs to the IN; and a beacon generator coupled to said slot allocator and configured to produce for transmission through the network a beacon message that advertises a second plurality of STRSs allocated as contention slots in which LNs may contend for association with the network; wherein the first plurality of STRSs is larger than the second plurality of STRSs.
 19. The apparatus of claim 18, wherein said node association interface is further configured to produce, when a LN associates with the network, a message for transmission to the LN that indicates a re-definition of the second plurality of STRSs for the LN such that at least one of the STRSs is changed to a SROS and a remainder of the STRSs are changed to STOSs, wherein said remainder is a majority of the STRSs.
 20. The apparatus of claim 19, wherein said node association interface is further configured to produce, when a further IN associates with the network, a message for transmission to the further IN that indicates allocation of a third plurality of STRSs to the further IN, wherein the third plurality of STRSs is larger than the second plurality of STRSs and smaller than the first plurality of STRSs, and wherein the IN interfaces between the further IN and the RN.
 21. The apparatus of claim 20, wherein said beacon generator is further configured to produce, for transmission through the network after the first and third pluralities of STRSs are allocated, a further beacon message that indicates allocation of one of at least one STOS and at least one SROS to each of at least one of the IN and the further IN, and wherein said slot allocator is further configured to allocate each said one of at least one STOS and at least one SROS in response to detection of a predetermined condition in the network.
 22. The apparatus of claim 19, wherein said beacon generator is further configured to produce, for transmission through the network after the first plurality of STRSs is allocated, a further beacon message that indicates allocation of one of at least one STOS and at least one SROS to the IN, and wherein said slot allocator is further configured to allocate said one of at least one STOS and at least one SROS in response to detection of a predetermined condition in the network.
 23. The apparatus of claim 18, wherein said node association interface is further configured to produce, when a further IN associates with the network, a message for transmission to the further IN that indicates allocation of a third plurality of STRSs to the further IN, wherein the third plurality of STRSs is larger than the second plurality of STRSs and smaller than the first plurality of STRSs, and wherein the IN interfaces between the further IN and the RN.
 24. The apparatus of claim 23, wherein said beacon generator is further configured to produce, for transmission through the network after the first and third pluralities of STRSs are allocated, a further beacon message that indicates allocation of one of at least one STOS and at least one SROS to each of at least one of the IN and the further IN, and wherein said slot allocator is further configured to allocate each said one of at least one STOS and at least one SROS in response to detection of a predetermined condition in the network.
 25. The apparatus of claim 18, wherein said beacon generator is further configured to produce, for transmission through the network after the first plurality of STRSs is allocated, a further beacon message that indicates allocation of one of at least one STOS and at least one SROS to the IN, and wherein said slot allocator is further configured to allocate said one of at least one STOS and at least one SROS in response to detection of a predetermined condition in the network.
 26. An apparatus for use as a RN in a TSCH wireless communication network that includes at least one IN that interfaces between the RN and a plurality of LNs, comprising: a slot allocator configured to allocate slots of time; a beacon generator coupled to said slot allocator and configured to produce for transmission through the network a beacon message that advertises a first plurality of STRSs allocated as contention slots in which LNs may contend for association with the network; and a node association interface coupled to said slot allocator and configured to produce, when a LN associates with the network, a message for transmission to the LN that indicates a re-definition of the plurality of STRSs for the LN such that at least one of the STRSs is changed to a SROS and a remainder of the STRSs are changed to STOSs, wherein said remainder is a majority of the STRSs.
 27. The apparatus of claim 26, wherein only one STRS is changed to a SROS.
 28. The apparatus of claim 26, wherein said node association interface is further configured to produce, when an IN associates with the network, a message for transmission to the IN that indicates allocation of a second plurality of STRSs to the IN, wherein said beacon generator is further configured to produce, for transmission through the network after the second plurality of STRSs is allocated, a further beacon message that indicates allocation of one of at least one STOS and at least one SROS to the IN, and wherein said slot allocator is configured to allocate said one of at least one STOS and at least one SROS in response to detection of a predetermined condition in the network.
 29. The apparatus of claim 26, wherein said node association interface is further configured to produce, when an IN associates with the network, a message for transmission to the IN that indicates allocation of a second plurality of STRSs to the IN, and to produce, when a further IN associates with the network, a message for transmission to the further IN that indicates allocation of a third plurality of STRSs to the further IN, wherein the third plurality of STRSs is larger than the first plurality of STRSs and smaller than the second plurality of STRSs, and wherein the IN interfaces between the further IN and the RN.
 30. The apparatus of claim 29, wherein said beacon generator is further configured to produce, for transmission through the network after the second and third pluralities of STRSs are allocated, a further beacon message that indicates allocation of one of at least one STOS and at least one SROS to each of at least one of the IN and the further IN, and wherein said slot allocator is further configured to allocate each said one of at least one STOS and at least one SROS in response to detection of a predetermined condition in the network.
 31. An apparatus for use as a RN in a TSCH wireless communication network that includes at least one IN that interfaces between the RN and a plurality of LNs, comprising: a slot allocator configured to allocate slots of time; a node association interface coupled to said slot allocator and configured to produce, when an IN associates with the network, a message for transmission to the IN that indicates allocation of a plurality of STRSs to the IN; and a beacon generator coupled to said slot allocator and configured to produce, for transmission through the network after said plurality of STRSs are allocated to the IN, a beacon message that indicates allocation of one of at least one STOS and at least one SROS to the IN, wherein said slot allocator is configured to allocate said one of at least one STOS and at least one SROS in response to detection of a predetermined condition in the network.
 32. The apparatus of claim 31, wherein the predetermined condition is more LNs than a predetermined threshold.
 33. An apparatus for use as a RN in a TSCH wireless communication network that has a hierarchical topology with a plurality of hierarchical levels and the RN interfaced to a plurality of LNs by a first IN at a relatively higher hierarchical level and a second IN at a relatively lower hierarchical level, comprising: a slot allocator configured to allocate slots of time; and a node association interface coupled to said slot allocator and configured to produce, when the first IN associates with the network, a message for transmission to the first IN that indicates allocation of a first plurality of STRSs to the first IN, and to produce, when the second IN associates with the network, a message for transmission to the second IN that indicates allocation of a second plurality of STRSs to the second IN; wherein the first plurality of STRSs is larger than the second plurality of STRSs.
 34. The apparatus of claim 33, including a beacon generator coupled to said slot allocator and configured to produce, for transmission through the network after the first and second pluralities of STRSs are allocated, a beacon message that indicates allocation of one of at least one STOS and at least one SROS to each of at least one of the first and second INs, wherein said slot allocator is further configured to allocate each said one of at least one STOS and at least one SROS in response to detection of a predetermined condition in the network. 